In U.S. Pat. No. 3,785,462 a lift mechanism of the scissor-jack type is described. This mechanism has a base platform and an elevatable platform interconnected by sets of spaced pairs of scissor arms pivoted intermediately the ends and pivotally connected to each platform such that relative movement between the respective platforms is accomplished by rotation of the scissor arms about their pivot. One side of the ends of the scissor arms is connected for a sliding motion along each of the platforms to accommodate the opening of the scissors while maintaining the platforms in spaced parallel positions. A combination of a cable and roller cam drive is employed for raising and lowering the platform with respect to the base. The cable is driven by a winch roller mounted on one side of the platform and reeved to pass over roller pulleys connected to each of the ends of scissors at one platform. The roller cam is positioned to drive the closed side of the scissor arms apart by working toward the pivot interconnecting each of the scissor arms with decreasing mechanical advantage during the first portion of the lifting cycle while the roller pulleys are pulled together with increasing mechanical advantage during the last portion of the lifting cycle.
A drawback in this design is that the contribution of the cable means guided over the roller while being pulled toward the pivot of the scissor arms varies with respect to the lifting distance. This means that no constant force over the whole lift distance is attainable. Furthermore, there is no provision for providing a determined, especially a constant and distinct, ratio between input and output forces as well as cable and platform displacement.
In U.S. Pat. No. 2,862,689 another scissor-jack type lifting device is disclosed. This device has a hydraulic pressure device with a cam roller as pushing means which rolls up a ramp, is lifted by that and rolls along a cam surface provided on one arm of the scissor arms. The cam roller is moved away from the scissor arm connection pivot in the lifting movement. This is a rather complicated design of a scissor-like lifting device and needs high power as the force for lifting. The platform is only attacking on one position and no constant ratio between input and output displacement, or between input and output force is disclosed.
Neither of the above-cited U.S. patents singularly nor in combination shows a possibility of designing a distinct, and over the whole lifting range, predetermined and constant input to output ratio of cable displacement and application forces. Furthermore, neither of them shows a cam follower riding on a cam surface toward a pivot connection of the arms when lifting is performed.